The present invention relates generally to the field of chemical treatment of comminuted cellulosic fibrous material (generally referred to herein as “chips”). In particular, the invention relates to heating chips in a pulping system and processing black liquor in the system, which includes an impregnation vessel and a chemical digesting vessel.
To produce pulp in a chemical pulping process, the lignin bonds between cellulose fibers are dissolved by “cooking” wood chips (or other cellulosic fibrous material) in a digester vessel containing chips and cooking liquor. White liquor is generally referred to as the virgin liquids and cooking chemicals before being introduced to the chip slurry. Black liquor, typically produced during the digesting process, is the combination of the lignin residue with water and the chemicals used for digesting the chips to form pulp. Black liquor is extracted from the pulping system, such as from the digester vessel. Black liquor can be introduced to the cooking process and removed during the cooking process.
FIG. 1 shows a conventional chemical digesting system 100, that processes wood chips. A continuous feed of wood chips is transported from a chip feed system (not shown) via line 101 to an impregnation vessel 102. Chips are pretreated with cooking liquor in the impregnation vessel 102. From the impregnation vessel 102, chips and liquor flow in a slurry through line 106 to the top inlet 103, e.g., top separator, of a chemical digester vessel 104. The top separator 103 in the upper internal region of digester vessel 104 feeds the chips to the inner chamber of the digester vessel.
As the chips and liquor flow through the top separator 103, a portion of the black liquor is extracted from the digester vessel via liquor circulation line 108. The liquor extracted from the top separator flows through an in-line drain 110, which directs the extracted liquor to a heat recovery unit 114 via line 112 or to a liquor circulation line 116.
A conventional liquor circulation line 116 transports a portion, e.g. 90%, of the liquor passing through the in-line drain 110 to the chip discharge at the bottom of the chip impregnation vessel 102 or to the chip slurry line 106. A liquor pump 118 may be used to move the liquor through the line 116 and to the chip slurry flowing from the impregnation vessel and to the digester vessel. The extracted liquor in line 116 is added to the pretreated chips discharged from the impregnation vessel and being transported via line 106 to the digester vessel.
The extracted liquor is introduced to the chip slurry in line 106 at or near the bottom of the impregnation vessel to increase the liquor content of the slurry for transport through the line. The liquor from line 116 is introduced to the chip slurry to reduce the concentration of chips in the slurry and facilitate chip transport through the chip slurry line 106. The slurry flows from the impregnation vessel through line 106 to the top of the digester vessel 104.
Conventionally, a small portion, e.g., 10%, of the extracted liquor flows via line 112 to the heat recovery system 114, e.g., the heat exchanger. The temperature of the extracted liquor flowing through the in-line drainer 110 and heat exchanger is, for example, about 110° C. (Celsius). Heat energy from the extracted liquor may be used in a heat exchanger to generate, for example, low pressure steam, e.g., at 1 bar, for use in the puling mill.
The liquor concentration L/W (ratio of liquor to chips by weight) in the impregnation vessel may be 2.5. In the chip slurry line 106, the liquor concentration (L/W) is increased by the addition of liquor (line 116) to the chips in the bottom of the impregnation vessel or an initial region of the chip slurry line 106. The addition of liquor and aid increase I the L/W in transporting the chips through the chip slurry line 106. When the slurry enters the digester vessel 104, the ratio of liquor to chips (L/W) is reduced by, for example, extracting liquor at the top separator.
Heat is often added to the chips in the digester vessel. Generally, a digester vessel operates at a higher temperature than does an impregnation vessel. For example, an impregnation vessel may operate at a temperature of 110° C. and the digester vessel may operate at temperature of 140° C. The chip slurry enters the top separator at a lower temperature then the temperature in the digester vessel. Heat, such as medium pressure steam via line 134, is added to the vessel to increase the temperature of the chips in the digester vessel. Medium pressure steam 146 is typically at a pressure of 10 to 12 bar and a temperature of 180° C. to 190° C. The temperature of the digester vessel tends to be lowest at the top inlet of the vessel and progressively increases in a downward direction through the vessel.
The digester vessel 104 generally includes multiple elevations of screens, including screens at upper elevations of the digester vessel (where upper refers to screens 120, 121 that are above the lower elevations of screens 122). Screen(s) 122 (generally referred to as wash screens) at lower elevations of the digester vessel may be near the bottom of the digester vessel, such as in the lower third (⅓) to one quarter (¼) of the digester vessel. Liquor is extracted through the upper elevation screen 120 and fed via liquor line 124 to the heat recovery system 114. Liquor to be recirculated through the digester is removed by upper elevation screen 121 and fed to a liquor recirculation line 126.
Other sources of liquor for circulation in the digester may include wash liquor (W.L.) 140 and a cold blow liquor system 142. The was liquor 140, cold blow liquor 142 and liquor extracted from screen 121 are combined in line 126 and pumped 127 to a heater 44. The liquor for recirculation is heated in the heater 144 that uses a steam source 146 for heat energy. Typically, the circulation liquor is reintroduced via line 126 to the vessel at a different, e.g. higher, elevation from which the liquor was extracted.
The liquor extracted from lower elevation(s) of screens 122 may be, via line 128, circulated to the digester at a higher elevation or discharged to the heat recovery system. The liquor from the lower elevation(s) of screens 122 tends to be hotter, e.g., 140° C., than liquor extracted from the top separator 103 (via line 108) and liquor extracted from screens 120 at upper elevations (via lines 124, 126). The hot liquor (line 128) from the lower region of the digester is recirculated to the top of the digester, via line 132 and pump 130, without adding heat to the liquor. Below the lower elevation screens and near the pulp discharge, cold blow, e.g., cooled wash liquor, is added to reduce the temperature of the pulp being discharged from the digester through line 145. The cold blow is provided from a source 142 of cooling liquor and passes through a cold blow cooler 143 and lines to direct the cold blow to the bottom of the digester.
Generally a large portion, e.g., 70%, of the liquor extracted via line 128 from the lower elevation screens 122 flows to the heat recovery system 114. For example, liquor extracted to line 128 from the lower elevation of screens 122 may be divided such that a first portion, e.g., 70% to 80%, of liquor flows via line 128 to the heat recovery system 114; a second portion, e.g., 10% to 18%, of the liquor is pumped 130 to line 126 for recirculation to the top of the digester vessel, and a third portion, e.g., 8% to 12%, of the liquor is pumped 130 via line 132 to recirculation line 126.
The excess black liquor, e.g. waste liquor, extracted from screens in the digester vessel and not reintroduced to the vessel or to the chip slurry line 106, flows through the heat recovery system 114. A portion of all streams of excess liquor, e.g., from in-line drainer and upper and lower extraction screens, tend to flow through the heat recovery system where heat energy from the excess liquor is converted to steam, typically low pressure steam, for other uses in the mill. Low pressure steam is typically at 1 bar or less at a temperature of about 100° C., such as in a range of 90° C. to 110° C.
To increase the temperature of the chips in the digester vessel, medium pressure steam 146 is often added to the digester vessel via steam line 136 and heater 144. The medium pressure steam comes in at a temperature of 180° C. to 190° C. and increases the temperature of the chips in the digester vessel to promote the chemical reactions for digestion, e.g., breaking the lignin that bonds together the cellulosic fibers in the chip. Medium pressure steam requires energy to generate. Further, evaporators are generally needed to remove condensate resulting from the steam injected into the digester vessel.
There is a long felt need to reduce the energy requirements for a pulping system, including systems having an impregnation vessel and digester vessel. In particular, there is a long felt need for techniques to add heat to the chips in the digester vessel, which improve the energy efficiency of the pulping system and reduce the need for steam from external sources such as steam source 146.